1. Field of the Invention
This invention relates to an insulated electric wire, and more particularly, to an insulated electric wire suitable for coils in electric devices such as motors, transformers and the like, and a coil using the insulated electric wire.
2. Description of the Related Art
Generally, insulated electric wires (enameled wires) have been widely used as coils in electric devices such as armatures, transformers and the like. These insulated electric wires are configured to comprise a metal conductor (conductor) having a cross section suitable for an application use or a shape of a coil (e.g. round or rectangular shape), and an insulating coating layer around a periphery of the conductor, which comprises a single layer or two or more layers of insulating coating film formed by applying around the conductor and baking an insulating varnish including a resin such as a polyimide, polyamide imide, polyester imide or the like dissolved in an organic solvent.
The electric devices such as armatures, transformers and the like are driven by inverter controlling. In such electric devices using inverter controlling, when the inverter surge (surge voltage) resulting from the inverter controlling is high, this inverter surge voltage may cause partial discharge in the insulated electric wires constituting the coils of the electric devices, and may degrade or damage the insulating coating film of the insulated electric wires.
As a method for preventing the insulating coating film from the degradation or damage due to the inverter surge voltage, insulated electric wires having a following configuration have been known. In such an insulated electric wire, an insulating coating film is formed by applying around a conductor and baking a polyamide-imide resin insulating varnish obtained by mixing an aromatic diisocyanate component having two or less aromatic rings with an aromatic imide prepolymer containing an aromatic diamine component having three or more aromatic rings and an acid component (refer to JP-A-2009-161683, for example). JP-A-2009-161683 describes that the use of such polyamide-imide resin insulating varnish provides the insulating coating film with a low relative permittivity, thereby provides the insulated electric wire with a high partial discharge inception voltage (PDIV).